High-carbon hot-rolled steel sheet and method for producing the same

ABSTRACT

A high-carbon hot-rolled steel sheet with excellent width-direction homogeneity is provided. The steel sheet contains 0.2% to 0.7% carbon, 0.01% to 1.0% silicon, 0.1% to 1.0% manganese, 0.03% or less phosphorus, 0.035% or less sulfur, 0.08% or less aluminum, and 0.01% or less nitrogen, and the balance is iron and incidental impurities. The structure is such that the average ferrite grain size of edge parts of the steel sheet is less than 35 μm, the average ferrite grain size of a part closer to the center of the steel sheet than the edge parts is less than 20 μm, and the average carbide grain size is 0.10 μm or more and less than 2.0 μm. The steel sheet is produced by roughly rolling the steel, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C., coiling the steel at a temperature of 550° C. or less, pickling the steel, and subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by a batch annealing method.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2007/065788, with an international filing date of Aug. 6, 2007 (WO 2008/020580 A1, published Feb. 21, 2008), which is based on Japanese Patent Application Nos. 2006-221885, filed Aug. 16, 2006, and 2007-200672, filed Aug. 1, 2007.

TECHNICAL FIELD

This disclosure relates to high-carbon hot-rolled steel sheets and methods for producing such steel sheets, and particularly to a high-carbon hot-rolled steel sheet with excellent width-direction homogeneity and a method for producing such a steel sheet.

BACKGROUND

High-carbon steel sheets used for tools and automotive parts (gears and transmissions), for example, are subjected to heat treatment, such as quenching and tempering, after punching. Recently, high-carbon steel sheet users, including tool and parts manufacturers, have been seeking simplification of a forming process for reduced costs by shifting from conventional parts processing based on cutting and hot forging of castings to that based on press forming of steel sheets (including cold forging). Accordingly, there is a strong demand for high-carbon steel sheets, used as raw materials, with hardenability and stability of complex forming. In addition, stability of mechanical properties has been highly demanded in terms of maintenance and management of press machines and molds.

In light of the above circumstances, some techniques have been studied for homogenization of mechanical properties.

Japanese Unexamined Patent Application Publication No. 9-157758, for example, proposes a method for producing a high-carbon steel strip by hot-rolling a steel, heating the steel to the ferrite-austenite interphase region at a predetermined heating rate, and annealing the steel at a predetermined cooling rate. According to that technique, the high-carbon steel strip is annealed at the Ac1 point or more, that is, in the ferrite-austenite interphase region, to form a structure in which coarse spheroidized cementite is homogeneously distributed in the ferrite matrix. Specifically, a high-carbon steel containing 0.2% to 0.8% carbon, 0.03% to 0.30% silicon, 0.20% to 1.50% manganese, 0.01% to 0.10% soluble aluminum, and 0.0020% to 0.0100% nitrogen, with the ratio of the content of soluble aluminum to that of nitrogen being 5 to 10, is hot-rolled, is descaled by pickling, is annealed in an atmosphere furnace containing 95% or more by volume of hydrogen, with the balance being nitrogen, in a temperature range of not less than 680° C. at a heating rate Tv (° C./hr) of 500×(0.01−N(%) as AlN) to 2,000×(0.1−N(%) as AlN) and a soaking temperature TA (° C.) of the Ac1 point to 222×C(%)²−411×C(%)+912 for a soaking time of 1 to 20 hours, and is cooled to room temperature at a cooling rate of not more than 100° C./hr.

Japanese Unexamined Patent Application Publication No. 11-80884, for example, proposes a production method in which a hot-rolled steel sheet containing 0.1% to 0.8% by mass of carbon and 0.01% or less by mass of sulfur is subjected to a first heating step in which the steel sheet is kept in a temperature range of Ac1−50° C. to less than Ac1 for 0.5 hour or more and is then continuously subjected to a second heating step in which the steel sheet is kept in a temperature range of Ac1 to Ac1+100° C. for 0.5 to 20 hours and a third heating step in which the steel sheet is kept in a temperature range of Ar1 −50° C. to Ar1 for 2 to 20 hours, where the temperature kept in the second step is shifted to that kept in the third step at a cooling rate of 5 to 30° C./h. That is, Japanese Unexamined Patent Application Publication No. 11-80884 employs the three-step annealing to provide a high-carbon steel sheet having an average ferrite grain size of 20 μm or more.

Japanese Unexamined Patent Application Publication No. 2003-73742, for example, proposes a method of hot rolling a steel containing 0.2% to 0.7% by mass of carbon so that its structure contains bainite in a volume percentage of more than 70% before annealing the steel to uniformly coarsen ferrite grains, thereby extremely softening the steel. This technique is characterized in that the steel is hot-rolled at a finishing temperature of (Ar3 transformation point−20° C.) or more, is cooled at a cooling rate of more than 120° C./s to a cooling termination temperature of 550° C. or less, is coiled at a coiling temperature of 500° C. or less, is pickled, and is annealed at an annealing temperature of 640° C. to Ac1 transformation point.

The above techniques, however, have the following problems.

According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-157758, a high-carbon steel strip is annealed at the Ac1 point or more, that is, in the ferrite-austenite interphase region, to form coarse spheroidized cementite, although it is difficult to stabilize the hardenability and workability of such coarse spheroidized cementite structure.

The technique disclosed in Japanese Unexamined Patent Application Publication No. 11-80884 would lower productivity and raise costs in practical operation because it involves a complicated annealing process.

According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-73742, a hot-rolled steel sheet containing bainite in a volume percentage of more than 70% is subjected to spheroidizing annealing to coarsen ferrite grains, thereby extremely softening the steel sheet. The above technique, however, has a problem in that because the steel sheet is hot-rolled at a finishing temperature of (Ar3 transformation point-20° C.) or more before being rapidly cooled at a cooling rate of more than 120° C./s, a temperature rise resulting from exothermic transformation after the cooling degrades the structural stability of the hot-rolled steel sheet. For hardness after the spheroidizing annealing, additionally, only the surfaces of samples have been evaluated according to Rockwell B-scale hardness (HRB). The steel sheet cannot be stably softened because coarse ferrite grains tend to be heterogeneously formed in the thickness direction after the spheroidizing annealing and result in variations in mechanical properties.

Thus, it could be helpful to provide a high-carbon hot-rolled steel sheet with excellent width-direction homogeneity which has stable hardenability and press formability and a method for producing such a high-carbon hot-rolled steel sheet without the need for a complicated production process. In particular, it could be helpful to stabilize the structure of a steel sheet near edges thereof.

SUMMARY

We studied the effect of the constituent composition, microstructure, and production conditions of a high-carbon steel sheet on its width-direction homogeneity. As a result, we found that specifying the average ferrite grain size of a steel sheet over the entire width thereof and the average carbide grain size is important to achieve excellent width-direction homogeneity. In addition, we found that controlling the average ferrite grain size of edge parts of a steel sheet, the average ferrite grain size of a part closer to the center of the steel sheet than the edge parts, and the average carbide grain size within the respective appropriate ranges ensures stable hardenability and press formability, thus providing a high-carbon hot-rolled steel sheet with excellent width-direction homogeneity.

Based on the above findings, a production method for controlling the above structure has been studied, and a method for producing a high-carbon hot-rolled steel sheet with excellent width-direction homogeneity has been established.

We thus provide:

-   -   A high-carbon hot-rolled steel sheet contains, in percent by         mass, 0.2% to 0.7% carbon, 0.01% to 1.0% silicon, 0.1% to 1.0%         manganese, 0.03% or less phosphorus, 0.035% or less sulfur,         0.08% or less aluminum, and 0.01% or less nitrogen, and the         balance is iron and incidental impurities. The steel sheet has         such a structure that the average ferrite grain size of edge         parts of the steel sheet is less than 35 μm, the average ferrite         grain size of a part closer to the center of the steel sheet         than the edge parts is less than 20 μm, and the average carbide         grain size is 0.10 μm or more and less than 2.0 μm, where the         edge parts of the steel sheet are regions extending from         positions 25 mm from both sides to positions 75 mm from both         sides in a width direction of the steel sheet in hot rolling.     -   [2] The high-carbon hot-rolled steel sheet in Item [1] above         further contains, in percent by mass, one or more of 0.005% to         0.5% molybdenum, 0.005% to 0.05% titanium, and 0.005% to 0.1%         niobium.     -   [3] A method for producing a high-carbon hot-rolled steel sheet         includes roughly rolling a steel having the composition         according to Item [1] or [2] above, finish-rolling the steel at         a finishing temperature of more than (Ar3+40° C.), cooling the         steel at a cooling rate of more than 120° C./s within two         seconds after the finish rolling to a cooling termination         temperature of more than 550° C. and less than 650° C., coiling         the steel at a temperature of 550° C. or less, pickling the         steel, and subjecting the steel to spheroidizing annealing at a         temperature of 670° C. to the Ac1 transformation point by a         batch annealing method.

In the present description, the percentages of the constituents of the steel are all based on mass.

A high-carbon hot-rolled steel sheet with excellent width-direction homogeneity which has stable hardenability and press formability is provided. The high-carbon hot-rolled steel sheet with excellent width-direction homogeneity can be produced without special annealing conditions. This results in a high yield and reduced costs in production.

DETAILED DESCRIPTION

A high-carbon hot-rolled steel sheet is characterized in that the constituent composition is controlled as described below and the structure is such that the average ferrite grain size of edge parts of the steel sheet is less than 35 μm, the average ferrite grain size of a part closer to the center of the steel sheet than the edge parts is less than 20 μm, and the average carbide grain size is 0.10 μm or more and less than 2.0 μm; these are the most important requirements. Thus, specifying the constituent composition, the metal structure (average ferrite grain size for each segment in the width direction), and the shape of carbide (average carbide grain size) and satisfying all these conditions provides a high-carbon hot-rolled steel sheet with stable hardenability and press formability in the width direction, including the edge parts.

The edge parts of the steel sheet refer to regions extending from positions 25 mm from both sides to positions 75 mm from both sides in the width direction of the steel sheet in hot rolling. In general, regions extending 75 mm from both sides in the width direction of a steel sheet tend to be supercooled. This tendency makes it difficult to control the temperature in these regions, thus leaving considerable structural variations. Regions extending 25 mm from both sides in the width direction of the steel sheet, on the other hand, are generally not subject to quality assurance or are removed by, for example, side trimming. Thus, an object, in which the regions extending from positions 25 mm from both sides to positions 75 mm from both sides in the width direction of the steel sheet are referred to as the “edge parts of the steel sheet,” is to improve the structure in these regions so that it is close to the structure near the center in the width direction of the steel sheet.

The above high-carbon hot-rolled steel sheet with excellent width-direction homogeneity is produced by roughly rolling a steel having the composition described below, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C., coiling the steel at a temperature of 550° C. or less, pickling the steel, and subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by a batch annealing method.

Thus, the production conditions in the hot finish rolling, the cooling after the finish rolling, the coiling, and the annealing are totally controlled.

Our steels and methods will now be described in detail.

First, the reasons for selecting the chemical constituents of the steel will be described.

(1) Carbon: 0.2% to 0.7%

Carbon is the most fundamental alloy element of carbon steel; its content greatly affects quenching hardness and the amount of carbide in an annealed state. A steel having a carbon content of less than 0.2% has insufficient quenching hardness for applications such as automotive parts. On the other hand, if the carbon content exceeds 0.7%, the steel strip has poor productivity and loses convenience in handling due to low toughness after the hot rolling. This results in unstable production, thus making it difficult to reduce costs. To provide a steel sheet with appropriate quenching hardness and press formability at low cost, therefore, the carbon content is 0.2% to 0.7%, preferably 0.2% to 0.5%.

(2) Silicon: 0.01% to 1.0%

Silicon is an element serving to improve hardenability. If the silicon content falls below 0.01%, the steel sheet lacks quenching hardness. If the silicon content exceeds 1.0%, on the other hand, the steel sheet has poor press formability because ferrite is hardened through solution hardening. In addition, carbide tends to be graphitized, thus impairing hardenability. To provide a steel sheet with appropriate quenching hardness and press formability, therefore, the silicon content is 0.01% to 1.0%, preferably 0.01% to 0.8%.

(3) Manganese: 0.1% to 1.0%

Manganese, like silicon, is an element serving to improve hardenability. This element is also important because it traps sulfur as MnS to prevent hot cracking of slabs. If the manganese content falls below 0.1%, the above effects are insufficiently provided, and the hardenability is significantly decreased. If the manganese content exceeds 1.0%, on the other hand, the steel sheet has poor press formability because ferrite is hardened through solution hardening. To provide a steel sheet with appropriate quenching hardness and press formability, therefore, the manganese content is 0.1% to 1.0%, preferably 0.1% to 0.8%.

(4) Phosphorus: 0.03% or less

The phosphorus content is 0.03% or less, preferably 0.02% or less, because phosphorus is segregated at grain boundaries and degrades ductility and toughness.

(5) Sulfur: 0.035% or less

Sulfur is an element that must be reduced because it forms MnS with Mn and degrades press formability and toughness after quenching; a lower sulfur content is preferred, although a sulfur content of up to 0.035% is acceptable. Hence, the sulfur content is 0.035% or less, preferably 0.030% or less.

(6) Aluminum: 0.08% or less

The aluminum content is 0.08% or less, preferably 0.06% or less, because an excessive amount of aluminum added results in a large amount of AlN being precipitated and degraded hardenability.

(7) Nitrogen: 0.01% or less

The nitrogen content is 0.01% or less because an excessive nitrogen content decreases ductility.

With the above essential elements added, the steel achieves the target properties, although one or more of molybdenum, titanium, and niobium may be optionally added besides the above essential elements added to suppress formation of proeutectoid ferrite during the cooling after the hot rolling and to improve hardenability. In this case, the effect of the addition may be insufficient if the amount of molybdenum added falls below 0.005%, if the amount of titanium added falls below 0.005%, or if the amount of niobium added falls below 0.005%. If the amount of molybdenum added exceeds 0.5%, if the amount of titanium added exceeds 0.05%, or if the amount of niobium added exceeds 0.1%, on the other hand, the effect becomes saturated, and increased costs result. In addition, the steel sheet may have poor workability because its strength is increased by, for example, solution hardening or precipitation hardening. Hence, if molybdenum, titanium, and niobium are added, their contents are 0.005% to 0.5%, 0.005% to 0.05%, and 0.005% to 0.1%, respectively.

The balance other than above is iron and incidental impurities. For oxygen, an example of an incidental impurity, its content is preferably reduced to 0.003% or less because it forms nonmetallic inclusions which adversely affect quality. Additionally, copper, nickel, tungsten, vanadium, zirconium, tin, and antimony may be contained within the range of 0.1% or less as trace elements that do not impair the effects and advantages.

Next, the structure of the high-carbon hot-rolled steel sheet with excellent width-direction homogeneity will be described.

(1) Average Ferrite Grain Size of Edge Parts of Steel Sheet: Less Than 35 μm

To homogenize the structure in the width direction, it is particularly important to suppress formation of coarse grains in the edge parts, which tend to be supercooled. Suppressing formation of coarse grains in the edge parts provides a uniform grain structure and therefore excellent press formability. That is, if the average ferrite grain size is not less than 35 μm, the steel sheet has unstable press formability because it forms a duplex grain structure containing coarse grains. To achieve stable press formability, therefore, the average ferrite grain size is less than 35 μm. To achieve stable press formability, additionally, it is desirable to minimize the difference in grain size between the edge parts of the steel sheet and the part closer to the center of the steel sheet than the edge parts (hereinafter referred to as the central part of the steel sheet); the difference between the edge parts of the steel sheet and the central part of the steel sheet is preferably 15 μm or less.

A steel sheet including edge parts with an average ferrite grain size of less than 35 μm is provided by controlling the temperature in the finish rolling and the cooling conditions, as described below. Specifically, a steel sheet including edge parts with an average ferrite grain size of less than 35 μm is provided by roughly rolling the steel, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), and cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C.

Thus, the formation of coarse ferrite grains, which often occurs particularly in the edge parts, can be prevented by avoiding low-temperature finishing after rough rolling and setting appropriate cooling conditions (cooled at a cooling rate of more than 120° C./s within two seconds to a cooling termination temperature of more than 550° C. and less than 650° C.).

(2) Average Ferrite Grain Size of Part Closer to Center of Steel Sheet Than Edge Parts (Central Part of Steel Sheet): Less Than 20 μm

The average ferrite grain size is an important factor that determines the stability of press formability. That is, the steel sheet has excellent workability if it has an average ferrite grain size of less than 20 μm and therefore contains uniform grains with few coarse grains. Hence, the average ferrite grain size of the central part of the steel sheet is less than 20 μm. The average ferrite grain size, on the other hand, is preferably more than 5 μm because formation of extraordinarily fine grains increases strength and can therefore cause problems such as a decreased mold life.

A steel sheet including a central part with an average ferrite grain size of less than 20 μm is provided by controlling the temperature in the finish rolling and the cooling conditions, as described below. Specifically, a steel sheet including a central part with an average ferrite grain size of less than 20 μm is provided by roughly rolling the steel, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), and cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C.

(3) Average Carbide Grain Size: 0.10 μm or More and Less Than 2.0 μm

The average carbide grain size is an important requirement because it greatly affects press formability, punchability, and quenching strength in a heat treatment step after press forming. Finer carbide grains ensure stable quenching hardness because they are more soluble in a heat treatment step after processing, although an average carbide grain size of less than 0.10 μm leads to poor press formability because of increased strength. Increasing the average carbide grain size, on the other hand, improves press formability, although carbide grains with an average carbide grain size of not less than 2.0 μm decrease quenching hardness because they are less soluble in a heat treatment step after processing. Accordingly, the average carbide grain size is 0.10 μm or more and less than 2.0 μm. The average carbide grain size can be controlled depending on the production conditions, particularly, the cooling conditions after the hot rolling, the coiling temperature, and the annealing conditions.

Next, a method for producing the high-carbon hot-rolled steel sheet will be described.

The high-carbon hot-rolled steel sheet with excellent width-direction homogeneity is provided by roughly rolling a steel having the above contents of chemical constituents, finish-rolling the steel at a desired finishing temperature, cooling the steel under desired cooling conditions, coiling the steel, pickling the steel, and subjecting the steel to desired spheroidizing annealing by a batch annealing method. These will be described below in detail.

(1) Finishing Temperature in Finish Rolling (Rolling Temperature)

If the finishing temperature in the hot rolling of the steel (final-path rolling temperature) is not more than (Ar3+40)° C., numerous shear bands are introduced in prior-austenite grains in portions of the edge parts of the steel sheet, thus increasing the number of nucleation sites for transformation; this results in formation of fine ferrite grains. Accordingly, coarse ferrite grains are often formed, particularly in the edge parts, during the spherical annealing, where the high grain boundary energy serves as its driving force. Hence, the finishing temperature is more than (Ar3+40)° C. In particular, a finishing temperature of more than (Ar3+80)° C. is preferred to more stably prevent formation of coarse ferrite grains and achieve more excellent width-direction homogeneity. Although the upper limit of the finishing temperature is not particularly specified, a finishing temperature of 1,000° C. or less is preferred because a high temperature exceeding 1,000° C. tends to cause scaling defects.

Accordingly, the finishing temperature in the hot rolling of the steel (final-path rolling temperature) is more than (Ar3+40)° C.

The Ar3 transformation point (° C.) can be determined by the following equation (1):

Ar3=910−310C−80Mn−15Cr−80Mo  (1)

where the element symbols denote the contents (percent by mass) of the respective elements. (2) Cooling: Cooling Rate of More than 120° C./s within Two Seconds after Finish Rolling

If the hot-rolled steel sheet is slowly cooled, a large amount of proeutectoid ferrite is formed because of a low degree of supercooling of austenite. If the cooling rate is not more than 120° C./s, a stable uniform grain structure is not achieved because a considerable amount of proeutectoid ferrite is formed and carbide is heterogeneously distributed after the annealing. Hence, the cooling rate after the hot rolling is more than 120° C./s, preferably 200° C./s or more. Although the upper limit of the cooling rate is not particularly specified, a cooling rate of up to 700° C./s is reasonable for a thickness of 3.0 mm in terms of the capability of existing equipment.

In addition, if the time from the finish rolling to the start of the cooling exceeds two seconds, a stable uniform grain structure is not achieved because, as in the above case, proeutectoid ferrite is significantly formed and carbide is heterogeneously distributed after the annealing. Hence, the time from the finish rolling to the start of the cooling is two seconds or less. For further structural stabilization, the time from the finish rolling to the start of the cooling is preferably 1.5 seconds or less, more preferably 1.0 second or less.

(3) Cooling Termination Temperature: More Than 550° C. and Less Than 650° C.

If the primary cooling termination temperature after the hot rolling is not more than 550° C., a fine bainite structure can be formed during the hot rolling step, particularly in the edge parts of the steel sheet, where the temperature tends to be lower. After the final annealing, this bainite structure forms a coarse ferrite grain structure; thus, a structure with width-direction homogeneity is not achieved. If the cooling termination temperature is not less than 650° C., on the other hand, a stable uniform grain structure is not achieved because a coarse ferrite-pearlite structure is formed during the hot rolling step and carbide is heterogeneously distributed after the annealing. Hence, the cooling termination temperature is more than 550° C. and less than 650° C.

(4) Coiling Temperature: 550° C. or Less

If the coiling temperature after the cooling exceeds 550° C., a stable uniform grain structure is not achieved because the resultant ferrite-pearlite structure is insufficiently fine and carbide is heterogeneously distributed after the final annealing. Hence, the coiling temperature is 550° C. or less. Although the lower limit of the coiling temperature is not particularly specified, a coiling temperature of 200° C. or more is preferred because the steel sheet is degraded in shape with decreasing temperature.

(5) Pickling: Carried Out

The hot-rolled steel sheet after the coiling is pickled for descaling before the spheroidizing annealing. The pickling may be carried out by a common method.

(6) Spheroidizing Annealing: Batch Annealing at Temperature of 670° C. to Ac1 Transformation Point

After the pickling, the hot-rolled steel sheet is annealed to sufficiently grow ferrite grains into uniform grains and to spheroidize carbide. Spheroidizing annealing is broadly divided into (1) a method of heating a steel sheet to a temperature slightly above the Ac1 transformation point before slowly cooling the steel sheet, (2) a method of maintaining a steel sheet at a temperature slightly below the Ac1 transformation point for an extended period of time, and (3) a method of repeatedly heating and cooling a steel sheet between temperatures slightly above and below the Ac1 transformation point. Of these, we employ method (2) above to facilitate both growth of ferrite grains and spheroidization of carbide. Accordingly, batch annealing is employed because the spheroidizing annealing requires an extended period of time. If the annealing temperature falls below 670° C., the steel sheet has poor workability because both growth of uniform ferrite grains and spheroidization of carbide are insufficient, thus resulting in an insufficiently uniform grain structure. If the annealing temperature exceeds the Ac1 transformation point, coarse grains tend to be formed in the edge parts of the steel sheet. Accordingly, the annealing temperature for the spheroidizing annealing is 670° C. to the Ac1 transformation point, preferably 670° C. to 710° C. The Ac1 transformation point (° C.) can be determined by the following equation (2):

Ac1=754.83−32.25C+23.32Si−17.76Mn+4.51Mo  (2)

where the element symbols denote the contents (percent by mass) of the respective elements.

Thus, the high-carbon hot-rolled steel sheet with excellent width-direction homogeneity is provided. The constituents of the high-carbon steel can be adjusted either using a converter or using an electric furnace. The high-carbon steel subjected to the constituent adjustment is processed into a steel slab, a type of steel material, either by ingot making and slabbing or by continuous casting. The steel slab is then hot-rolled, where the slab is preferably heated to 1,300° C. or less to avoid surface degradation due to scaling. Alternatively, a continuous casting slab may be subjected to direct rolling as is or with its temperature being maintained to prevent a temperature decrease. Also, in the hot rolling, the finish rolling may be carried out with the rough rolling omitted. In addition, the material being hot-rolled may be heated by heating means such as a bar heater or an edge heater to maintain a sufficient finishing temperature in the edge parts of the steel sheet. After the coiling, furthermore, the temperature of the coil may be maintained using a slow-cooling cover, for example, to facilitate spheroidization or to reduce hardness.

After annealing, temper rolling is carried out if necessary. The conditions of the temper rolling are not particularly limited because it does not affect hardenability.

The reason why the high-carbon hot-rolled steel sheet thus produced combines hardenability with excellent press formability is as follows. The average ferrite grain size greatly affects the homogeneity of mechanical properties, which serves as a measure of press formability. The press formability is improved by forming uniform grains in the structure and limiting the content of coarse ferrite grains. The average carbide grain size, on the other hand, greatly affects hardenability. If carbide grains are coarse, unsolved carbide tends to remain after solution treatment followed by quenching, thus decreasing quenching hardness. From the above viewpoints, a high-carbon hot-rolled steel sheet with excellent width-direction homogeneity which has both hardenability and press formability can be provided by specifying the constituent composition, the metal structure (average ferrite grain size), and the shape of carbide (average carbide grain size) and satisfying all these conditions.

Examples

The steels containing the chemical constituents shown in Table 1 were subjected to continuous casting, and the resultant slabs were heated to 1,250° C., were hot-rolled under the conditions shown in Table 2, were pickled, and were subjected to spheroidizing annealing by a batch annealing method under the conditions shown in Table 2, thus producing hot-rolled steel sheets with a thickness of 4.0 mm.

TABLE 1 (% by mass) Steel No. C Si Mn P S Sol. Al N Others Ar3 Ac1 A 0.22 0.19 0.71 0.011 0.008 0.031 0.0038 Trace 785 740 B 0.33 0.20 0.68 0.009 0.008 0.029 0.0033 Trace 753 737 C 0.35 0.21 0.74 0.011 0.008 0.031 0.0038 Mo: 0.01 742 735 D 0.34 0.19 0.72 0.010 0.007 0.030 0.0036 Ti: 0.029 747 736 Nb: 0.013 E 0.66 0.22 0.72 0.009 0.011 0.028 0.0031 Trace 648 726

TABLE 2 Cooling Cooling Steel Finishing initiation Cooling Termination Coiling Spheroidizing sheet Steel Ar3 Ac1 temperature time rate temperature temperature annealing No. No. (° C.) (° C.) (° C.) (s) (° C./s) (° C.) (° C.) conditions Remarks 1 A 785 740 890 1.0 220 580 530 700° C. × 20 hr Invention example 2 A 785 740 900 0.8 200 600 540 690° C. × 30 hr Invention example 3 B 753 737 860 0.4 180 560 510 690° C. × 20 hr Invention example 4 B 753 737 870 0.6 200 580 530 700° C. × 20 hr Invention example 5 C 742 735 870 1.0 180 620 550 670° C. × 20 hr Invention example 6 C 742 735 850 0.4 200 580 530 700° C. × 30 hr Invention example 7 D 747 736 860 1.1 190 610 520 670° C. × 20 hr Invention example 8 D 747 736 870 0.5 210 580 540 700° C. × 30 hr Invention example 9 E 648 726 830 0.6 160 600 530 680° C. × 20 hr Invention example 10 E 648 726 850 0.5 220 620 550 690° C. × 20 hr Invention example 11 A 785 740 900 0.9 80 600 540 700° C. × 30 hr Comparative example 12 B 753 737 860 3.0 220 580 550 690° C. × 20 hr Comparative example 13 B 753 737 880 0.9 200 680 550 700° C. × 20 hr Comparative example 14 C 742 735 850 0.4 180 600 540 650° C. × 30 hr Comparative example 15 C 742 735 880 1.1 160 600 580 680° C. × 20 hr Comparative example 16 D 747 736 860 3.0 200 560 520 700° C. × 20 hr Comparative example 17 E 648 726 850 0.9 50 580 540 700° C. × 30 hr Comparative example 18 E 648 726 860 1.6 220 530 520 680° C. × 20 hr Comparative example 19 B 753 737 800 0.5 210 560 520 700° C. × 20 hr Invention example 20 B 753 737 805 0.5 200 550 530 700° C. × 20 hr Invention example 21 B 753 737 810 0.4 210 600 520 710° C. × 20 hr Invention example 22 B 753 737 820 0.6 220 560 510 700° C. × 20 hr Invention example 23 B 753 737 835 0.6 190 560 510 710° C. × 20 hr Invention example 24 B 753 737 845 0.4 230 580 520 710° C. × 20 hr Invention example 25 B 753 737 855 0.5 220 590 530 700° C. × 20 hr Invention example 26 B 753 737 790 0.5 220 580 540 710° C. × 20 hr Comparative example 27 B 753 737 775 0.5 230 580 530 710° C. × 20 hr Comparative example 28 B 753 737 760 0.4 200 590 520 710° C. × 20 hr Comparative example 29 B 753 737 755 0.6 200 580 530 700° C. × 20 hr Comparative example

Next, a sample was taken from each hot-rolled steel sheet thus produced and was evaluated for the average ferrite grain size of the edge parts of the steel sheet, the average ferrite grain size of the central part of the steel sheet, and the average carbide grain size, and the material hardness, reflecting structural conditions, was also measured. The methods and conditions for the respective measurements are as follows.

Average Ferrite Grain Size

The average ferrite grain size was measured from the structure of each sample in a cross section taken across the thickness in the rolling direction by optical microscopy according to JIS G 0552 (1998) “Methods of Ferrite Grain Size Test for Steel.” Specifically, the grain size number G was determined by the cutting method described therein, the number, m, of crystal grains per cross-sectional area of 1 mm² was calculated from m=2^((G+3)), and the average crystal grain size, d, was determined by the following equation (1). The average grain size was determined by averaging the grain sizes measured in a sufficient number of fields of view so that at least 3,000 ferrite grains were cut:

d(μm)=1000/√m  Equation (1).

Average Carbide Grain Size

The carbide grain size was measured by polishing and corroding a cross section, taken across the thickness in the rolling direction, of each sample and imaging its microstructure by scanning electron microscopy. The average grain size was determined by averaging the grain sizes of at least 500 carbide grains.

Material Hardness

The average hardness was determined by Rockwell hardness (HRB) measurement at three points in different positions in the width direction (the center and the positions 25 mm from the edges) in the surface of each sample. The average hardness thus determined was used to determine the difference in hardness between the central part and edge parts of the steel sheet (ΔHRB=(hardness of edge parts of steel sheet)−(hardness of central part of steel sheet)).

The results of the above measurements are shown in Table 3.

TABLE 3 Average ferrite Average ferrite Average Steel grain size of grain size of carbide Hardness of material surface (HRB) sheet Steel central part of edge parts of grain size Central part Edge parts of steel sheet No. No. steel sheet (μm) steel sheet (μm) (μm) of steel sheet (25 mm from edges) ΔHRB Remarks 1 A 16 30 0.9 75 72 −3 Invention example 2 A 18 33 0.9 74 72 −2 Invention example 3 B 15 28 1.1 80 79 −1 Invention example 4 B 16 25 1.2 79 78 −1 Invention example 5 C 14 29 1.1 82 80 −2 Invention example 6 C 15 26 1.0 81 79 −2 Invention example 7 D 13 29 1.0 82 80 −2 Invention example 8 D 14 26 0.9 82 80 −2 Invention example 9 E 10 21 1.3 91 89 −2 Invention example 10 E 11 21 1.5 90 89 −1 Invention example 11 A 24 17 2.2 72 76 4 Comparative example 12 B 23 18 2.5 79 82 3 Comparative example 13 B 35 30 3.5 76 79 3 Comparative example 14 C 17 45 0.4 79 65 −14 Comparative example 15 C 24 16 2.5 76 82 6 Comparative example 16 D 24 16 2.6 77 82 5 Comparative example 17 E 23 10 2.3 87 91 4 Comparative example 18 E 11 45 1.3 91 78 −13 Comparative example 19 B 15 32 1.1 80 71 −9 Invention example 20 B 16 34 1.2 79 69 −10 Invention example 21 B 13 29 1.3 86 80 −6 Invention example 22 B 18 28 1.1 80 73 −7 Invention example 23 B 19 27 1.3 81 76 −5 Invention example 24 B 18 26 1.3 81 79 −2 Invention example 25 B 19 25 1.0 77 74 −3 Invention example 26 B 14 40 1.1 80 68 −12 Comparative example 27 B 16 48 1.1 80 66 −14 Comparative example 28 B 15 53 0.8 79 58 −21 Comparative example 29 B 14 58 0.9 77 58 −19 Comparative example

In Table 3, Steel Sheet Nos. 1 to 10 and 19 to 25 are examples in which the production conditions fell within our range and the structure was such that the average ferrite grain size of the edge parts of the steel sheet was less than 35 μm, the average ferrite grain size of the central part of the steel sheet was less than 20 μm, and the average carbide grain size was 0.10 μm or more and less than 2.0 μm. It was found that the high-carbon hot-rolled steel sheets produced in the examples, in which no coarse grains were formed in the edge parts of the steel sheets, were stable in terms of hardness in the width direction, namely, not more than 10 points in the difference in material hardness (ΔHRB) between the central parts and edge parts of the steel sheets. In particular, the steel sheets produced in the examples (Steel Sheet Nos. 1 to 10 and 23 to 25) in which the finishing temperature was more than (Ar3+80° C.) were more stable in terms of hardness in the width direction, namely, not more than 5 points in ΔHRB. It was also found that the steel sheets produced in the examples contained fine carbide grains. As a result, high-carbon hot-rolled steel sheets with stable hardenability and press formability were produced.

Steel Sheet Nos. 11 to 18 and 26 to 29, on the other hand, are comparative examples in which the production conditions were outside our range. Steel Sheet Nos. 14, 18, and 26 to 29 were outside our range because numerous coarse grains were formed in the edge parts of the steel sheets and the average ferrite grain size was not less than 35 μm. As a result, the differences in hardness between the central parts and edge parts of the steel sheets exceeded 10 points; they did not achieve homogeneous mechanical properties in the width direction and were unstable in terms of press formability. For Steel Sheet Nos. 11 to 13 and 15 to 17, the average ferrite grain sizes of the central parts of the steel sheets and the average carbide grain sizes thereof were outside our range because of the large average ferrite grain sizes of the central parts of the steel sheets, meaning that the structure contained insufficiently uniform grains, and the large average carbide grain sizes. As a result, both hardenability and press formability were unstable.

INDUSTRIAL APPLICABILITY

The high-carbon hot-rolled steel sheet with excellent width-direction homogeneity can readily be processed into parts of complicated shapes, including transmission parts such as gears, under a low load. The steel sheet can therefore be used for a wide variety of applications, particularly for tools and automotive parts (gears and transmissions). 

1. A high-carbon hot-rolled steel sheet comprising, in percent by mass, 0.2% to 0.7% carbon, 0.01% to 1.0% silicon, 0.1% to 1.0% manganese, 0.03% or less phosphorus, 0.035% or less sulfur, 0.08% or less aluminum, and 0.01% or less nitrogen, the balance being iron and incidental impurities, and having a structure wherein average ferrite grain size of edge parts of the steel sheet is less than 35 μm, the average ferrite grain size of a part closer to a center portion of the steel sheet than the edge parts is less than 20 μm, and average carbide grain size is 0.10 μm or more and less than 2.0 μm, and the edge parts of the steel sheet are regions extending from positions 25 mm from both sides of the steel sheet to positions 75 mm from both sides in a width direction of the steel sheet in hot rolling.
 2. The high-carbon hot-rolled steel sheet according to claim 1, further comprising, in percent by mass, one or more of 0.005% to 0.5% molybdenum, 0.005% to 0.05% titanium, and 0.005% to 0.1% niobium.
 3. A method for producing a high-carbon hot-rolled steel sheet, comprising: roughly rolling a steel having the composition according to claim 1, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C., coiling the steel at a temperature of 550° C. or less, pickling the steel, and subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by batch annealing.
 4. A method for producing a high-carbon hot-rolled steel sheet, comprising: roughly rolling a steel having the composition according to claim 2, finish-rolling the steel at a finishing temperature of more than (Ar3+40° C.), cooling the steel at a cooling rate of more than 120° C./s within two seconds after the finish rolling to a cooling termination temperature of more than 550° C. and less than 650° C., coiling the steel at a temperature of 550° C. or less, pickling the steel, and subjecting the steel to spheroidizing annealing at a temperature of 670° C. to the Ac1 transformation point by batch annealing. 